Oil contaminants in compressed air, even in small concentration levels at sub-ppm levels, can be disastrous and health threatening for a vast variety of industries such as food and beverage processing, pharmaceutical manufacturing and packaging, chemical and petrochemical processing, semiconductor and electronics manufacturing, the medical sector, automotive paint spraying, textile manufacturing, amongst several others.
The International Standards Organization (ISO) established the 8573 compressed air quality standard in 1991 to govern compressed air system component selection, design and measurement. ISO 8573 is a multi-part standard, with Part 1 classifying contaminant type and assigning air quality levels, and Parts 2 through 9, define testing methods to accurately measure a full range of contaminants within the end user's facility. ISO 8573.1 identifies three primary contaminant types as prevalent in a compressed air system: Solid particulates, water and oil (in both aerosol and vapour form). Each is categorized and assigned a quality class ranging from class 0, the most stringent, to Class 9, the most relaxed.
The present invention comprises novel photoacoustic sensor system that in certain embodiments has the ability to, in real time, detect and measure oil contamination in vapour, aerosol and liquid form in compressed air systems in accordance with class 0 requirements of the ISO 8573 standard. The photoacoustic sensor system may in certain embodiments achieve class 0 accuracy according to the ISO 8573 standard as tested and verified by the Danish National Metrology Institute (Dansk Fundamental Metrologi A/S—“DFM”) and the Dutch National Metrology Institute (VSL).
High purity compressed air is a frequently used source of energy/power in industrial environments and a key element in a large variety of industries. High purity compressed air is for example used in pneumatics for driving different types of valves and actuators in the automotive and process industries. High purity compressed air is also used as a gas duster for cleaning electronic components in the electronics and semiconductor industry. Air jet weaving and air jet spinning use compressed air for pick insertion and yarn consolidation in the textile industry. Other applications of compressed air include automotive industry spray paint booths, tobacco industry air washers, hospital surgery and nursery rooms, breathing air in hospitals, photographic film manufacturing plants, and clean rooms. In addition to high purity compressed air requirements in the above mentioned industries, clean compressed air is vital for maintaining cleanliness and hygiene in food and beverage industries where contaminations can lead to severe health issues. Compressed air provides the energy source for pneumatic conveyers that transport liquids, powders and moisture sensitive product throughout the plants. Compressed air provides power for pneumatically operated tools and equipment that renders meat products, aerates liquids and mixes granular ingredients. It is used to package, wrap, seal, palletize and label food products prior to storage or shipment. The wide-spread use of compressed air has resulted in increased contamination vulnerability and the industry has as a consequence been subject to strict regulation and standards. In this context, ISO 8573 is the most important international standard. The standard specifies quality requirements for all compressed air—and part 1 specifies the purity classes of compressed air with respect to particles, water and oil. The standards vastly increase the operating and maintenance costs of compressed air systems along with the tremendous risks associated with an air contamination.
A compressed air system can have various different types of contaminations, and these can generally be attributed to the following sources:                1. The quality of air being drawn into the system: Atmospheric air in an industrial environment typically contains 140 million dirt particles for every cubic metre of air. 80% of these particles are less than 2 microns in size and are too small to be captured by the compressor intake filter and other inline filters, therefore passing directly into the compressed air system. These often contain hydrocarbon particles and aerosols.        2. The type and operation of the air compressor: The air compressor itself can also add contamination, from wear particles to coolants and lubricants. Most air compressors use oil in the compression stage for sealing, lubrication and cooling. During operation, lubricating oil is carried over into the compressed air system as liquid oil and aerosols. This oil mixes with water vapour in the air and is often very acidic, causing damage to the compressed air storage and distribution system, production equipment and final product.        3. Compressed air storage and distribution systems: The air receiver and system piping are designed to store and distribute the compressed air. As a consequence they also store the large amounts of contamination drawn into the system. Additionally, piping and air receivers also cool the moist compressed air forming condensate that causes damages and corrosion.        
Failure to detect oil contamination, or other types of contaminants, in due time can cause significant health problems and lead to severe economic impacts. Examples are:                1. Increase in plant maintenance costs: Corrosion within compressed air storage vessels and the air distribution system, blocked or damaged valves, filters, cylinders, air motors and air tools, damaged production equipment.        2. Unscheduled plant shutdown: Caused by equipment and part failures, leading to loss of production capacity.        3. Product contamination: Leading to loss of production batches and economic losses.        4. Reduced Product Quality: Contaminated products if released into the market will not only result in reduced brand image, and conflicts with regulatory bodies, but in the case of food processing and pharmaceutical industries could severely affect public health.        5. Risk to Human Life with Severe Civil Liabilities: Oil contaminants in compressed air for patients in hospitals can have life threatening impacts for patients.        
The present photoacoustic sensor system may be configured to detect various types of oil contamination and therefore eliminate at least some of these risks and impacts. To achieve these goals, the present photoacoustic sensor system may for example be integrated with SCADA and Distributed Control Systems of the plants.
Hence, the present photoacoustic sensor system and methodology may be adapted for the detection of oils in aerosol, vapour and liquid form in compressed air. Some embodiments of the photoacoustic sensor system may be configured to detect target molecules comprising oil contaminants in compressed air with ISO 8573 class 1, or better, sensitivity. The present photoacoustic sensor system and methodology may also be adapted to detect other types of target molecules such as hydrocarbons, non-hydrocarbons and trace gasses. While photoacoustic spectroscopy technologies are well known they have generally not been satisfactory in terms of accuracy, sensitivity, reproducibility and traceability. The same applies for other technologies, for example, thermal and photo ionization technologies. A key reason is that these prior art technologies are prone to background noise.
The present photoacoustic sensor system and methodology is capable of eliminating, or at least markedly suppressing, this noise while enhancing various important detection parameters. The present photoacoustic sensor system and methodology may include one or several new research-based sensing principles and sophisticated new photonics and optical spectroscopy with noise cancellation technology and flow noise immunity designs, optimised insulation, coatings and sampling etc. as discussed in further detail below.
U.S. Pat. No. 5,986,546 describes an oil contamination detection assembly for a vehicular pneumatic brake system having a supply of air contaminated with oil. The principle behind this detection is based on the change in resistance of a resistor when accumulating a layer of oil contaminating the air. Since this is used in a vehicular maintenance system, it does not conform to ISO standards, and this principle produces detection that is far less accurate than industrial requirements.
U.S. Pat. No. 5,730,942 describes a system for measuring foreign substances such as oil in a gas stream, by introducing a probe that is based on change in resistance/conductivity. This patent is a contact type measurement that has relatively poor sensitivity and repeatability and hence does not conflict with this innovation.
U.S. Pat. No. 4,732,861 describes a method of detecting oil aerosol in an air flow, by passing the air flow through a space between an electrode and an electrically conductive catalyst, causing electrical discharge between the electrode and the catalyst so as electrostatically to precipitate oil from the air flow on to the catalyst, terminating the discharge, stopping the air flow, heating the catalyst in substantially stagnant air to a temperature at which catalytic combustion of the precipitated oil occurs, and sensing heat generation due to said catalytic combustion to produce an output signal indicative thereof.
U.S. Pat. No. 7,343,781 describes a system for detecting fine liquid like oils and particles in a gas system comprising of a compressor and at least two gas handling devices. This patent does not describe any sensor element but only describes the system and arrangement for real-time measurement.
EP 2 373 991 describes a system based on the photo ionisation technology.
The present photoacoustic sensor system and methodology may be a valuable instrument for the implementation of critical and continuous monitoring of contamination of compressed air or air. The present photoacoustic sensor system may for example comply with certain requirements for microbial monitoring and enabling a proactive intervention.